1. Field of the Invention
The present invention relates to a phase shift mask (PSM), and more particularly, to a phase shift mask simultaneously having a structure of a selective chromeless phase shift mask and a structure of an alternating phase shift mask.
2. Description of the Prior Art
In circuit making processes, a lithographic process has not only been a mandatory technique but also played an important role in limiting feature size. Only by a lithographic process can a wafer producer precisely and clearly transfer a circuit pattern onto a semiconductor wafer. In a lithographic process, a designed pattern, such as a circuit pattern or a doping pattern, is created on one or several photo masks, then the pattern on the mask is transferred by light exposure, with a stepper and scanner, onto a semiconductor wafer.
It is an important issue for solving resolution of the lithographic process due to the device sizes of the semiconductor industry are being reduced. There are primarily two methods in the prior art for improving resolution. One method involves using short wavelengths of light to expose a photoresist layer on the semiconductor wafer. Short wavelengths of light are desirable as the shorter the wavelength, the higher the possible resolution of the pattern. Another method involves the use of a phase shift mask (PSM) to improve the resolution of the pattern transferred to the semiconductor wafer. Please refer to FIG. 1, which is a structural diagram of a prior art alternating phase mask 10. As shown in FIG. 1, a fully opaque material such as chrome is used in a non-transparent region 12 of the alternating phase shift mask 10, and the non-transparent region 12 is flanked by transparent regions 14, 16. Both of the transparent regions 14, 16 are made of quartz. The thickness of the transparent region 14 is approximately 2280 Angstroms less than that of the transparent region 16. Therefore, light passing through the transparent region 14 has a phase shift of 180 degrees relative to light passing through the thicker transparent region 16, which results in destructive interference. Consequently, during the lithographic process, a dark unexposed region falls on an area of a photoresist layer and is located below the non-transparent region 12 of the alternating phase shift mask 10.
However, the alternating phase shift mask 10 has to perform a double exposure lithography technique employing a trim mask to complete pattern transferring. Additionally, the alternating phase shift mask 10 suffers from transmission imbalance occurred in the phase shift region, i.e. the transparent region 14, and the non-phase shift region, i.e. the transparent region 16.
Therefore, a selective chromeless phase shift mask is developed. Please refer to FIG. 2, which is a structural diagram of a prior art selective chromeless phase shift mask 20. As shown in FIG. 2, the selective chromeless phase shift mask comprises a transparent region 22 made of quartz, and the transparent region 22 is flanked by two transparent regions 24, 26 also made of quartz. The transparent region 22 is thicker than both the transparent regions 24, 26, which causes a 180 degree phase shift in light passing through the transparent regions 24, 26. In other words, the transparent regions 24, 26 are phase shift regions, and the transparent region 22 is a non-phase shift region. Because of this 180 degrees phase difference, there is destructive interference at the phase boundaries of the phase shift regions 24, 26 and the non-phase shift region 22. Consequently, during the lithographic process, a dark unexposed region falls on an area of a photoresist layer and is located below the non-phase shift region 22 of the selective chromeless phase shift mask 20.
However, with the increase of packing density of devices, a pitch between each device is reducing. Unfortunately, when the pitch is less than twice of a width of the device and forms a dense pattern, light of 0 degree phase shift and light of 180 degrees phase shift cancel out. Therefore, the prior art selective chromeless phase shift mask fails to transfer the dense pattern.